Optical trace gas sensors measure the concentration of gas species by absorption spectrometry. Direct absorption spectroscopy through silicon waveguides and other photonic platforms with large refractive index contrast can, however, suffer from strong etalons due to partial reflection from intermediate scattering points on the waveguide, as well as absorbing features (particularly C—H stretch absorption in the near-infrared) from particulate contaminants adsorbed onto the waveguide surface. Unwanted spectral features arising from the aforementioned fringe and contamination result in peak-to-peak fractional amplitude variations of about 1×10−2, resulting in poor detection limits at long integration times. For reference, typical free-space optical spectroscopic trace gas detection systems are capable of detecting peak-to-peak fractional amplitude variations on the order of about 1×10−5.
Conventional etalon subtraction (difference between sample and zero-gas trace) is insufficient for precision trace-gas monitoring in high index contrast integrated photonic platforms, particularly given the thermal dependence of fringing which causes slow changes in fringe free-spectral range over time.
Given the deleterious impact of these strong etalons, improved data analysis techniques are needed to restore trace gas concentration accuracy and precision.